Most inkjet printheads have, for manufacturing reasons, a limited length, which limits the length of the printouts that can be done along the axis of revolution of the object to be printed.
An inkjet printhead comprises rows of nozzles on its lower face. The spacing between each nozzle is equal to the definition, in dots per inch (DPI).
The lower surface for example includes four rows of 90 nozzles each. The two outermost rows are for example spaced apart by 2.82 mm, while the nozzles in the four rows are spaced apart in pairs by 0.0705 mm, which leads to a definition of 360 DPI.
The length of a row of nozzles is for example approximately 72.1 mm. The width of the lower face is for example 17.2 mm.
The width of the lower face corresponds to the thickness of the printhead.
Thus, using a single printhead, by causing the object to be printed to pass below the printhead, it is possible to obtain a printout with a length of 72.121 mm.
In the event one wishes to print a flat object over extensions exceeding 72.121 mm in a transverse direction perpendicular to the movement direction of the object, it is known to place printheads behind one another in the transverse direction, such that the last nozzle of one of the heads is situated at 0.0705 mm from the first nozzle of the following head. As many printheads are used as are required by the extension of the printing, the printheads generally being placed in staggered rows to account for their thickness. In this position, the median planes of the printheads are separated from one another by at least the width of the lower face of the printheads, or 17.2 mm. In practice, the median planes are separated from one another by at least 22 mm to account for the supports of the printheads.
When this arrangement of the printheads in staggered rows is used to print on a revolution object, the object is rotated around its axis of revolution successively below each printhead, such that the entire revolution surface is successively presented in front of each printhead. Thus, if the printing station includes two printheads, the object to be printed performs at least two revolutions.
Added to the time necessary to perform those two revolutions is the time to transfer the object from one printhead to the other, over a distance of at least 22 mm. During that transfer, it also rotates the object such that the beginning of the printing happens at the second printhead.
To perform this printing, either the movement of the object is reversed during the transfer over 22 mm, or the object continues to be rotated at the same speed and in the same direction, which amounts to performing the 22 mm transfer at the same time as that necessary to perform another revolution. The second of these solutions is generally chosen, since the first solution involves significant decelerations and accelerations that may lead to slipping of the object relative to its object carrier, which is detrimental to the precision of the printing. Therefore in practice, the installation in staggered rows causes the object to perform three revolutions, and therefore decreases the printing pace by three.
In order to print revolution objects, it is possible to place the printheads of a same printing station in a radial configuration from the axis of revolution of the object, as described in document FR 10 58 717.
In this configuration, the median planes of the printheads pass through the axis of revolution of the object. The object is not moved from one printhead to the next in translation in the same printing station, which makes it possible to save time. However, the radial configuration of the printheads leads to increasing the separation between the printing stations. For example, to print on an object with a diameter of 40 mm, the distance between the printing stations goes from 22 mm to 75 mm.
Furthermore, the radial configuration of the printheads creates, at the object, a volume bulk that requires moving the object away from the printheads to bring it from one printing station to the next. It is generally considered that, for good definition of the printing, the distance between the lower face of a printhead and the object must be less than approximately 0.9 mm.
For example, if an object having a diameter of 40 mm is placed at 0.9 mm from the printheads of a printing station positioned radially, that object, after printing, must be transversely offset by approximately 5 mm during its movement toward the following printing station so as not to collide with the printheads that have just printed or those of the following printing station. Then, the object must be brought transversely closer to the following printing station to once again be situated at 0.9 mm from the printheads. This leads to additional transverse movements of the object, and therefore a reduction in the printing rhythm.